Salmonella typhimurium
is a Gram-negative, rod-shaped bacterium (Figure
1). When grown on ordinary agar, the organism
forms spherical smooth colonies about two to
four millimeters in diameter. When grown on Hektoen
enteric agar, colonies are
bluish-green with black centres, indicating that
the species does not ferment
lactose (common to many Salmonella
species). The use of Hektoen enteric agar is designed for
detection of these types of bacteria.

Figure 1.
Under a very high magnification of 15000 X, this
colorized scanning electron micrograph reveals
the presence of numbers of clustered
Gram-negative Salmonella typhimurium
bacteria, which had been grown in a pure
culture.

Movement is accomplished with
a peritrichous flagella arrangement, but does
not produce endospores. Some of the metabolic
features that can help identify the bacteria
include being lactose negative, citrate
positive, lysine decarboxylase positive, gelatin
hydrolysis negative, and the production of
hydrogen sulfide. S. typhimurium can be
found on certain food items, and also within an
infected host's fecal matter. Stool or food
samples thought to contain the pathogen are
isolated and analyzed. The bacteria is incubated
at 37°C and allowed to grow for 18 to 24 hours
before being plated out onto selective agar. The
broth most commonly used for selective
identification of S. typhimurium is
Rappaport-Vassiliadis soy broth.

This pathogen is infamously
known for causing food poisoning in
humans, resulting in gastroenteritis in
humans and other mammals. Food poisoning often occurs when a
person consumes uncooked meat or comes in
contact with the fecal matter of an infected
individual. In nature, S. typhimurium
may survive several years in the soil. Once the
organism enters the host, symptoms are not noted
until 12 to 24 hours have passes. The most
common symptoms include: diarrhea, vomiting, and
fever; this lasts between two to five days.
S. typhimurium infects the host by
penetrating the intestinal mucosa and migrating
to the spleen and liver where it causes
systematic disease (Rosenberger et al.,
2000).

The most common risk factor
is ill prepared food, such as undercooked beef,
pork, and lamb. Handling uncooked meat may
contaminant hands, plates, kitchenware, and
grounding the meat may also cause multiplication
of the bacteria, increasing the risk of
infection. Recall that grounding the meat
increases its surface area; this exposes S.
typhimurium, an aerobic bacteria, to
greater amounts of atmospheric oxygen, and in
turn, providing it with its desired growth
environment that is oxygen-enriched. Moreover,
food poisoning normally will clear up on its own
in healthy individuals. However,
immunocompromised, elderly, young children,
organ-transplant recipients, people receiving
treatment for cancer and pregnant woman are at
greater risk of infection, as it may cause
typhoid-like symptoms if left untreated.

S. typhimurium
evades the immune system of its host by first
being taken up by the epithelial layer of the
small intestine or macrophages of the small
intestine and replicating within specialized
vacuoles. The pH level within these vacuoles
that are produced as a result of engulfment
induces the production of bacterial gene
products required for its survival in the
epithelial cell of phagocyte (Rosenberger et
al., 2000). Moreover, as with all
Gram-negative bacteria, the outer membrane
contains lipopolysaccharide (LPS), which
contains the lipid A endotoxin. Once this is
released, it can cause shock, a condition that
creates an excessive immune reaction that can be
fatal. Another virulence factor is
enterochelin, which is a chelating agent
created by these bacteria. Iron is required for
bacterial growth, and this agent allows iron to
be sequestered from the host body for use by the
bacteria only (Yancey et al., 1979). If
the bacteria can obtain the iron, its virulence
will increase within the host.

In order to prevent food
poisoning caused by species of the genus
Salmonella, attenuated Salmonella
vaccine strains have been developed by
introducing mutations into the bacterium that
diminish its overall growth. Several strains
have been attenuated (deactivated or killed)
through the introduction of auxotrophic
(nutritional) defects, which slow the growth of
the bacterium by preventing it from synthesizing
needed biomolecules such as amino acids. A more
recent strain contained mutated DNA adenine
methyltransferase (Dam) gene, which is essential
for DNA repair (Pier et al., 2004).